1. Field of the Invention
The present invention relates to a misfire discriminating method for an engine. More particularly, it relates to a method of discriminating the misfire of an engine for each cylinder wherein the misfire condition of each cylinder is discriminated from the momentum change rate between cylinders. Also, it relates to such a method wherein the misfire judgement level of each cylinder is set with reference to a revolution fluctuation arising when fuel has been cut off, and wherein the misfire condition of each individual cylinder is discriminated on the basis of the misfire judgement level.
2. Description of the Prior Art
In general, it is ideal for producing a stable output that combustion in a multicylinder engine undergoes the same process every cycle. In the multicylinder engine, however, the combustion is liable to deviate due to the synergy of such drawbacks as listed below:
(1) Nonuniformity in the distribution rate of intake air attributed to the complication of the shape of an intake pipe, the interferences of the intake air among cylinders, etc.
(2) Some differences among the combustion temperatures of the individual cylinders attributed to cooling routes.
(3) Manufactural dispersions in the volumes of the combustion chambers of the individual cylinders, the shapes of pistons, etc.
(4) Slight discrepancies of the air fuel ratios of the individual cylinders caused by unequal fuel injection quantities ascribable to the manufactural errors of injectors, etc.
Heretofore, the combustion fluctuations among the cylinders have been suppressed to the minimum by the air fuel ratio controls and ignition timing controls of the individual cylinders. In a recent high-performance engine tending toward a higher output and a lower fuel cost, however, when any of the injectors, ignition plugs etc. has degraded or broken down, intermittent misfire is liable to arise and to incur lowering in the output.
Even when the intermittent misfire has occurred in one cylinder in the multicylinder engine, a driver often drives an automobile without noticing the misfire. Besides, it is difficult to judge during the drive whether the cause of the misfire is merely temporary or is the degradation or the like of any of the injectors, the ignition plugs etc.
Therefore, according to the official gazette of Japanese Patent Application Laid-open No. 258955/1986 by way of example, a comparison is made between the difference of the minimum value and maximum value of the r. p. m. of an engine corresponding to a cylinder at the previous combustion stroke and the difference of the minimum value and maximum value of the engine r. p. m. corresponding to the cylinder at the present combustion stroke. The combustion condition of the pertinent cylinder is discriminated, depending upon whether or not the discrepancy between the compared values falls within a preset reference value. In a case where abnormal combustion has occurred in excess of a predetermined number of times, a misfire is judged, and warning is given.
With the prior-art technique, the combustion fluctuation of each cylinder is obtained from the the differences between the minimum engine r. p. m. and maximum engine r. p. m. values of the combustion stroke cylinder. During combustion, however, the engine speed rises abruptly, and the engine is loaded relatively heavily, so that the fluctuation of an acceleration increases. Accordingly, it is difficult to specify the maximum value of the engine r. p. m., and a precision error developing at the judgement of the misfire enlarges.
Meanwhile, the combustion characteristics of an engine deviate, not only among cylinders, but also among individual engines on account of the manufactural errors of components, etc.
When the reference value for comparing the revolution fluctuations is set as an absolute value as in the prior-art technique, it changes relatively every engine due to the deviation of the combustion characteristics of the individual engines, and it becomes difficult to precisely grasp abnormal combustion in some of the engines.
In an engine having a small number of cylinders, the combustion intervals between the cylinders are comparatively long, and hence, the difference of the revolution fluctuations is large. Therefore, even when the reference value is set as the absolute value, the deviation of the combustion characteristics of the individual engines do not exert great influence on the misfire judgement. In contrast, in an engine having a large number of cylinders, the combustion intervals are short, and the difference of the revolution fluctuations decreases to that extent. Therefore, when the judgement level (the reference value) is set as the absolute value beforehand, the deviation of the combustion characteristics of the individual engines affects the precision of the misfire judgement greatly.
Especially in a high speed region, the fluctuation difference becomes small. Therefore, when the judgement level changes relatively every engine, the precise misfire judgement becomes very difficult.
By way of example, with a technique in the official gazette of Japanese Patent Application Laid-open No. 82534/1984, a differentiated r. p. m. value .DELTA.N#i (.DELTA.N#i=NH#i-NL#i) which is the difference between the instantaneous engine r. p. m. NL#i of each of cylinders #i (i=1.about.4 in a four-cylinder engine) before a combustion stroke and the instantaneous engine r. p. m. NH#i thereof after the combustion stroke is obtained every cylinder, and the differentiated r. p. m. value .DELTA.N#i of each cylinder #i is subsequently compared with the average value .DELTA.NA of the differentiated r. p. m. values .DELTA.N#i of all the cylinders #i, thereby to grasp the combustion condition of each cylinder #i.
With the prior-art technique, however, a reference value is the average differentiated r. p. m. value .DELTA.NA of all the cylinders, which in itself is always prone to fluctuate depending upon the combustion conditions. When the average differentiated r. p. m. value .DELTA.NA of all the cylinders has fluctuated, that combustion condition of each cylinder which is estimated with reference to the average differentiated r. p. m. value .DELTA.NA of all the cylinders comes to involve the combustion condition factors of the other cylinders in the case of setting this value .DELTA.NA, and it becomes difficult to precisely grasp the misfire of each cylinder.
Meanwhile, with a technique wherein as disclosed in the official gazette of Japanese Patent Application Laid-open No. 268956/1988, misfire is decided from the difference of momentums before and after combustion (a "revolving circumferential difference" in the official gazette), the misfire is simply judged on condition that the momental difference is minus. In, for example, the running of an automobile at a constant speed, however, the momental difference sometimes becomes minus under the influence of the friction of an engine in spite of normal firing (normal combustion), and the misfire might be erroneously decided.